DISEASES OF PEARS 



1041 



earlier stairs of decay a sticky 

 gray substance oozes from the 

 lenticels, collecting in drops 

 that later run down the side of 

 the fruit. This substance ma y 

 he found in moist, warm 

 weather to be oozing from the 

 affected portion of the tree 

 (Fig. 328). Late* it dries 

 down to a linn amber-colored 

 mass that may become almost 

 black. 



('aiise of the Disease 

 This disease is caused by 

 <nie of the lower plant organ- 

 isms called bacteria. The bo- 

 tanical name of the organism 

 causing fire blight is Bacillus 

 (imylovoms. Like other bac- 

 teria, it is exceedingly small and, while a large number together 

 can be seen with the naked eye, a single specimen is-^visible only 

 with the aid of a high-power lens. Even then it is seen indistinctly 



unless one is accustomed to looking 

 through a microscope. These bacilli 

 have an oblong body with from two to 

 four long, whiplike flagella that enable 

 them to swim about in the sap of 

 affected tissues. They increase in 

 number by dividing at the middle into 

 two, and then growing to normal size. 

 This division may take place every 

 half hour as long as conditions are 

 favorable for their development. 



FIG. 321. TWIG BLIGHT. NOTE 

 THAT IT BEGAN AT THE VERY TIP 



FIG. 322. BLOSSOM BLIGHT. 

 XOTE CANKER ABOUT 

 BASE OF SPUR 



Life History of iFire Blight 

 In a more or less inactive condition, 

 these bacteria pass the winter in hold- 



